1. Field of the Invention
This invention generally relates to fiber reinforced composites and particularly relates to preforms having woven strips of material used in reinforced composite materials, which can be woven flat and formed into their final shape, the final shape having reinforcement in two or more directions.
2. Description of the Prior Art
The use of reinforced composite materials to produce structural components is now widespread, particularly in applications where their desirable characteristics are sought of being light in weight, strong, tough, thermally resistant, self-supporting and adaptable to being formed and shaped. Such components are used, for example, in aeronautical, aerospace, satellite, recreational (as in racing boats and automobiles), and other applications.
Typically such components consist of reinforcement materials embedded in matrix materials. The reinforcement component may be made from materials such as glass, carbon, ceramic, aramid, polyethylene, and/or other materials which exhibit desired physical, thermal, chemical and/or other properties, chief among which is great strength against stress failure. Through the use of such reinforcement materials, which ultimately become a constituent element of the completed component, the desired characteristics of the reinforcement materials, such as very high strength, are imparted to the completed composite component. The constituent reinforcement materials typically, may be woven, knitted or braided. Usually particular attention is paid to ensure the optimum utilization of the properties for which the constituent reinforcing materials have been selected. Usually such reinforcement preforms are combined with matrix material to form desired finished components or to produce working stock for the ultimate production of finished components.
After the desired reinforcement preform has been constructed, matrix material may be introduced to and into the preform, so that typically the reinforcement preform becomes encased in the matrix material and matrix material fills the interstitial areas between the constituent elements of the reinforcement preform. The matrix material may be any of a wide variety of materials, such as epoxy, polyester, vinyl-ester, ceramic, carbon and/or other materials, which also exhibit desired physical, thermal, chemical, and/or other properties. The materials chosen for use as the matrix may or may not be the same as that of the reinforcement preform and may or may not have comparable physical, chemical, thermal or other properties. Typically, however, they will not be of the same materials or have comparable physical, chemical, thermal or other properties, since a usual objective sought in using composites in the first place is to achieve a combination of characteristics in the finished product that is not attainable through the use of one constituent material alone. So combined, the reinforcement preform and the matrix material may then be cured and stabilized in the same operation by thermosetting or other known methods, and then subjected to other operations toward producing the desired component. It is significant to note at this point that after being so cured, the then solidified masses of the matrix material normally are very strongly adhered to the reinforcing material (e.g., the reinforcement preform). As a result, stress on the finished component, particularly via its matrix material acting as an adhesive between fibers, may be effectively transferred to and borne by the constituent material of the reinforcement preform.
The increased use of composite materials having such fiber preform reinforcements in aircraft fuselage barrels has led to the need for composite window frames. Traditional metallic window frames cannot be used for this application because of differences between the coefficients of thermal expansion of the composite fuselage and the metallic frame. In addition, parasitic barrier plies must be used to eliminate corrosion problems that can exist when some composites and metals are in contact. These barrier plies increase cost of production as well as the overall weight.
Aircraft window frames 10, for example such as that shown in FIG. 1, tend to have the shape of an oval with the major axis of the frame curved to accommodate the cylindrical shape of the fuselage. The cross sectional shape of the window frame 10, such as that shown in FIG. 2, for example, is usually uniform. However, the shape can include complicating features such as an upstanding leg 20 at the outer edge, and/or what are called “joggles” 15 that facilitate sealing the window to the main body of the aircraft. The upstanding leg 20 is a particularly difficult feature to incorporate into a composite design because of the oval shape of the frame 10. Fabricating this feature with conventional fabric or tape requires the use of darts to form the curved shape. These darts, however, increase the hand labor required to fabricate the preform and reduce the strength of the resulting composite.
Solutions that do not require the upstanding leg have been developed, and are currently being used on aircrafts such as the Boeing 787 (See U.S. Patent Publication No. 2008/0078876 and 2008/0169380, for example). This more simple geometry can be fabricated using a compression molding process along with a sheet molding compound such as Hexcel Corporation's HexMC®. However, for structures that require an upstanding leg, there is still a need for a method that can provide continuous fiber in the body as well as the upstanding leg and that may lead to reduced weight and/or improved performance of the frame.
WO 2005115728, for example, relates to a method for making a window frame for installation in the exterior shell of an aircraft. The structure includes an outer flange, an inner flange, and a vertical flange arranged perpendicular to and between these two flanges.